This invention pertains generally to nonconsumable welding electrodes and more particularly to nonconsumable, composite electrodes designed for use with gas tungsten arc welding torches.
Gas tungsten arc welding processes achieve coalescence by heating the weld zone with an arc struck between a tungsten electrode and the workpiece. To prevent oxidation the heated weld zone, the molten metal and the non-consumable electrode are shielded from the atmosphere by an inert gas stream which is directed from the electrode holder, commonly referred to as the welding torch.
Most commercial torch designs employ a collet for supporting and transferring weld current to the tungsten electrode. The electrode generally extends from the collet, axially through a surrounding gas nozzle and projects a short distance beyond the nozzle orifice. The shield gas is conveyed through the nozzle, along the electrode and out the orifice to the weld. U.S. Pat. No. 2,794,898 noted that the current carrying capacity of a tungsten electrode is limited by the amount of resistance heat generated by the conductance of weld current through the electrode and the heat radiated from the arc. The prior art proposed the use of thoriated tungsten to increase the emissability (efficiency) of the electrode for a given amperage and thus enable a reduction in applied amperage to reduce the heating effects of the electrode and improved water cooling to dissipate the heat in the electrode to increase the electrodes current carrying capacity. The above-noted patent recognized further that the electrode itself was a major source of heat which led to the electrodes failure and limited its current carrying capability. The patent proposed a composite electrode having a central tungsten core and an outer copper sheath swaged to the tungsten. The copper sheath was employed as a low resistance current carrier to transport the weld current from the collet to the tungsten weld tip. The addition of the copper sheath, initially and under a short extension and low duty cycle, will significantly reduce the heat that would have been otherwise generated along the length of tungsten short circuited by the copper. While the addition of the sheath in theory provided an improvement in some applications, it failed to overcome the damaging effects of electrode heating under a prolonged duty cycle or extended electrode length (long I.sup.2 R heat inducing path) typical in narrow groove applications. Experience has shown that under such conditions the difference in coefficients of thermal expansion of the copper and tungsten under welding operating temperatures will result in upsetting the copper sheath fit, heavily oxidizing the interface and substantially reducing the effective conductivity of the copper current path. In deep, narrow grooves, which can extend for example over four inches in depth and have side walls inclined at less than six degrees, the radiant heat from the arc contributes more significantly to the degradation of the electrode. As a result of the severe heating environment of a deep, narrow groove the copper sheath described in Patent 2,794,898 expands away from the tungsten creating an increase in the electrical resistivity of the contact surface area at the tungsten/copper interface. Thus under these conditions a significant portion of the weld current is either redirected through the tungsten or forced to jump the high resistance gap across the copper/tungsten interface. In not having the capability to carry the waste heat of I.sup.2 R heating and radiant pick-up away from the electrode's tip geometry, that geometry cannot be sustained for any appreciable length of time.
Accordingly, an improved electrode design is desired that will increase the operating life and current carrying capacity of a nonconsumable, tungsten electrode in narrow groove applications.